It is known to employ power MOS transistors, especially power MOSFETs or power IGBTs, as controllable switches for switching of electrical loads.
FIG. 1 shows a circuit arrangement with a power MOS transistor M, configured as a MOSFET, which is employed as a switch, and whose load path(drain-source section) is connected in series with a load Z between terminals for a first and second supply potential Vs, GND. The MOSFET is connected here as a low-side switch, i.e., the load path is connected between the load and the negative power supply potential or reference potential GND.
A fundamental goal in the controlling of a power MOS transistor is to achieve smooth switching slopes, after the MOS transistor is turned on or off, for the current flowing through the load or the voltages imposed across the load and the transistor, so that temporary changes in the load current, and thus an electromagnetic interference radiation, will be reduced.
FIG. 2 illustrates the time curves of the load current IL and the drain-source potential of the MOSFET M for a resistive load Z and with the MOSFET M controlled by a driver circuit 10 as shown in FIG. 1. This driver circuit, responding to a control signal S1, after a switch-on time t1, charges the internal gate-source capacitance Cgs of the MOSFET M, likewise shown in the figure, across a first current source 12 with a constant charging current I12 up to a maximum value Vgs_max, to trigger the MOSFET M into the conducting condition. After a switch-off time t4, the driver circuit 10 discharges the gate-source capacitance Cgs across a second current source 13 with a constant discharge current I13 down to zero, in order to block the MOSFET M.
In this type of driving, the curve of the gate-source potential Vgs between times t2 and t3 after the switch-on time t1 or between times t5, t6 after the switch-off time t4 has regions with very slight gradients, known as “Miller plateaus”, being caused by charging effects of the gate-drain capacitance (not shown). The gate-source potential Vgs in the region of the Miller plateau lies in the region of the threshold voltage of the MOSFET.
The time curve of the load current Iz across the MOSFET M and the time curve of the drain-source potential Vds shows that these curves have comparatively steep edges at the beginning and at the end of the Miller plateaus.
To reduce the EMI radiation when switching a power MOSFET, DE 198 55 604 C1 describes how to charge and discharge the gate-source capacitance of the MOSFET during the switching on and off process with different charging and discharging currents, each of them having a constant amplitude.
DE 102 40 167 A1 describes a method whereby the gate charging current for conductive triggering and the gate discharging current for blocking of a MOSFET is increased as the voltage across the load decreases.
WO 00/27032 describes a circuit arrangement for controlling a power MOSFET, which lowers by stages the gate discharging current during the switch-off process with decreasing voltage across a load connected in series with the power transistor (see FIG. 4).
DE 198 36 577 C1 describes a method for controlling a low-side switch, configured as a MOSFET, in a bridge circuit. In this method, a difference between the maximum voltage present across the low-side switch, which corresponds to a power supply voltage, and a voltage which is momentarily present across the low-side switch is determined. Then the ratio of this difference and the power supply voltage is formed, and the gate-source voltage of the MOSFET is adjusted in this method as a function of this ratio.